PROJECT SUMMARY/ABSTRACT A growing body of research indicates that siblings have marked influence on adolescents' alcohol and other substance use. Results from genetically informed designs reveal that concordance between siblings in these domains exceeds the influence of shared genetics and shared environments, including parenting, suggesting that sibling similarities arise via some form of social influence. Despite strong evidence that siblings are unique socializers of substance use, research on the mechanisms driving these associations is underdeveloped. There is a critical need to identify those processes that influence adolescents' decisions regarding substance use. The lack of mechanistic understanding precludes informed development of family-based behavioral intervention strategies. Thus, rooted in ecological and family systems frameworks, we propose an integrative theoretical framework of sibling influence and identify the micro- and global-social influence processes that account for sibling similarities and differences. Additionally, taking a developmental perspective, we will identify the dual trajectories and reciprocal associations between older and younger siblings' substance use behaviors and related cognitions from early through late adolescence. This advance is critical, as previous work has only considered top-down (i.e., older to younger sibling) socialization. Testing ecological principles, we will also investigate the degree to which community characteristics moderate the influence of sibling socialization and youth's substance use more generally. The sample will include 600 sibling pairs and their residential parents (~2220 participants). Using address based sampling strategies, we will recruit families that include a focal older sibling in grades 8-10 (~13-16 years of age) and a sibling up to three grades younger (~12-15 years of age). Using an accelerated longitudinal within-family design with planned missingness, participants will be followed over the course of two years (i.e., three annual assessments). Two data collection methods will be employed at each assessment. First, annual internet-based computer assisted self-interviews (CASI) with parents and youth will assess participants' alcohol and other substance use and related cognitions, psychosocial functioning, and peer, parent and sibling relationship qualities. Second, a series of internet-based diary interviews will be completed by younger and older siblings. Over four consecutive weekends, these interviews will assess the content of siblings' daily interactions as well as the contexts and companions of their activities (including substance use). In general, the aims will be tested using latent variable modeling (LVM). This flexible analytic procedure is advantageous as it can handle both diary and longitudinal data which are nested within individuals as well as data from siblings which are further nested within families. Furthermore, LVM permits the efficient testing of mediating and moderating effects as well as growth trajectories and reciprocal effects. By identifying specific mechanisms of sibling influence, this study will provide important information about new targets for family-based interventions to reduce adolescent substance use.